On the subject of underrated tools I would have to include a fid or marlin spike.
A sailor used to working with
lines and cordage well knows how important such a tool is and yet there are many outdoorsmen who use cordage just as much but are completely unaware of how useful it can be.
This one is inspired by Inuit art and is made from an old piece of whalebone carved into a seals head gripping a silver ring.
This fid, or others like it, have been used to loosen jammed knots, open holes for threading through, splicing ropes and on
occasion as a toggle to secure a temporary knot for hauling.
I have lost count of the occasions that it has proved a useful tool in a pinch and I certainly do not begrudge the couple of ounces it weighs.